It is known to harden the surfaces of the cams of a valve-lifting camshaft for a motor-vehicle internal-combustion engine by passing a heat source over the surfaces. Normally as described in commonly owned U.S. patent applications Ser. Nos. 940,199 and 940,200 both filed Sept. 7, 1978 a tungsten-inert-gas (TIG) torch is used which is held adjacent the cam as the camshaft is rotated. In this manner a camshaft of grey cast iron can have its cam surfaces hardened to a high degree with relative ease.
It is also known from German utility model No. 7,702,409 filed Jan. 28, 1977 by the assignee of the instant application to axially relatively reciprocate the TIG torch and the camshaft as the camshaft is rotated, so that the torch heats the surface of the cam along an undulating path. This path is normally confined to the central two-thirds of the cam surface in accordance with the above-cited U.S. patent applications.
This German utility model also describes how the heat source constituted by the TIG torch is maintained at a fixed axial spacing from the cam surface it is acting on by means of a lathe-type motion-copying system. A master camshaft is held adjacent the camshaft to be hardened, and the two shafts are synchronously rotated about parallel axes, with the various lobes of the one shaft lying in predetermined angular positions to the lobes of the other shaft. A follower arrangement engaging the lobes of the cams of the master or template camshaft controls the radial displacement of the respective TIG torch so that same rests a slight distance off the respective camshaft. Such an arrangement allows the cam surfaces to be hardened accurately and with excellent production speed.
The disadvantage of this system is that the very locations on the cams which require the most hardening, that is the radially projecting lobes, often are hardened least effectively. This is due to the fact that these lobes pass more rapidly under the respective torch, since the angular rotation rate is constant and these lobes project radially further than the rest of the cams so that they pass more rapidly under the heat source. As a result the treatment of these projecting lobes is substantially less.
It has been suggested to overcome this disadvantage by using a relatively slow angular rotation speed or relatively high reciprocation frequency to ensure good hardening of even these lobes. The disadvangtage of this is that the amount of treatment time devoted to the rest of the cams is far in excess of what is needed, particularly since it is the lobes that are subject to the most wear on such a camshaft.